In my Patent U.S. Pat. No. 4,500,103 for a HIGH EFFICIENCY BICYCLE FRAME, very large diameter frame tubing is used in a bicycle to resist relatively large torsional and bending forces to produce a bicycle which is very light in weight yet extremely rigid and which, at the same time, provides an extremely good ride. In my Patent U.S. Pat. No. 4,621,827, I disclose a bicycle in which the chainstay tubes are made of unequal rigidity and made in such a way so as to increase the power train efficiency by reducing the magnitude of frame deflection caused by chain stress. The present invention is directed to improvements in the steering and front fork assembly head set bearing and handlebar stem of a bicycle.
The front fork of bicycles typically have been steel with about one inch steerer (e.g., one inch outside diameter steerer post in steel). That is what the headset bearings and all the headset pieces were made to accommodate and the one inch size was limiting in steel. The steel steerer uses a fairly thick wall near the crown in order to make the fork strong enough.
In the bicycle described in my patent U.S. Pat. No. 4,621,827, the head tube had an outside diameter of about 1.42 inches and an inside diameter of about 1.180 and a center section wall thickness of about 0.065 inches. In order to fit in one inch bearing size constraint in aluminum, a solid bar had to be used and it still is not strong enough because of the small diameter size.
In the past, on mountain bikes and on some road bikes, others have started promoting larger headset sizes with 11/4 inch steering tubes. This is still made of steel in order to make the forks more rigid for better cornering control but they are still essentially about the same weight or heavier. There have been suggestions of aluminum forks. These use a conventional headset and headset bearing units. Hence, the front fork and headset assembly of a bicycle has been the heavy end of the bike and it has been the end that gets the most shock.
There has been introduced to the market a number of front forks which do not have curved blades but which have instead straight blades and there is controversy in the bicycling art concerning whether these straight blades provide harsher riding forks or not. The present invention uses straight blades.
The wheel axle is typically offset forward of the steering axis in order to obtain desirable handling. This offset is called the fork rake. The present invention uses a fork rake of about 11/2 inches.
Headset bearing failures are a frequent problem in off-road bicycles. The repeated impacts of off-road use brinell the bearings, loosen the bearing housings in the head tube and the fork crown, loosen and damage the threaded adjustment mechanism. Because of angular misalignment tolerances necessary for inexpensively machined steerer crowns, head tubes and adjusting threads, the traditional bearing assemblies use a cup and cone system, where the radius of curvature of the balls is much smaller than that of at least one of the raceways. This allows the bearing to tolerate angular misalignment and substantially reduces the contact area of the balls, compared to the Super Conrad style bearing--with raceways closely fitted to the balls. The rigidity of the point contact style bearings is thus substantially lower than that of the torque tube type bearing and the load carrying capacity is very much lower. This invention is able to fully utilize the advantages of double-sealed aircraft torque tube type bearings by machining the outside diameter of the steering tube for direct fit and adhesive bonding of the headset bearings to the external surfaces of the steering tube and raceway seats in the head tube, insuring accurate alignment. The ends of the head tube are also precision bored for alignment, and also benefit from direct fit. The threaded adjustment of traditional headsets is another source of trouble. The threads weaken the thin wall steering tube and can break there, especially if the handlebar stem is clamped inside the threads.
The invention results in a bicycle front end which does not require frequent adjustment or services with far greater durability, and is directed to improvements in the front fork and headset and steering assembly and is particularly directed to the utilization of very large diameter aluminum tubing, a unique headset bearing assembly. According to the invention, the fork blades are greater than about 11/2 inch in diameter at the top and about 11/8inch at the fork ends down at the tip. They are, in the preferred embodiment, rounded all the way: they are straight for a predetermined distance and then they taper and have a wall thickness proportional to the forces or loading at specific locations on the blades.
Each blade is mitered at the crown end at an angle and a specially configured crown tubing is mitered to fit up against the blade. It is very difficult to bend the big tube easily and have a tight radius so in the disclosed embodiment the large diameter aluminum tubing is miter cut and welded.
Furthermore, instead of using a conventional headset, a steer tube of about 15/8 inch diameter was utilized and the part that goes up through the bearings is about 1-9/16 inch so that the steer tube is actually about 1-9/16 inch. The outside diameter of the bearings is about two inches so that the head tube has a diameter of about 21/4 inches at the top and bottom to provide raceway seats to fit the bearings and the bearings are pressed fit and adhesively bonded right to the head tube and to the steering tube. In this present application, the steerer tube has been machined to locations where the stress or forces are less and tapered to the bearing seats where the tube wall is thickened.
This extremely large diameter head tube along with the large diameter steering tube or post provides a more positive control in rough conditions and is significantly stiffer in both torsional stiffness and fore and aft stiffness and side-to-side stiffness than traditional one inch steerer forks and has essentially the same rigidity as the more recent larger 11/4 inch forks. Moreover, the weight is significantly less than any prior art fork and steering assembly having equivalent rigidity.
The crown piece according to this invention is much larger than that used in a regular fork. With this in mind, if standard headset bearings are used, the front end of the bike is elevated in the air e.g., the stack height is exaggerated. Hence, instead of using a conventional headset, this invention utilizes about 15/8 inch steer tube and the part that goes through the bearings is about 1-9/16 inch diameter so that in effect a machined to about 1-9/16 inch steer tube up through the bearings and forms a shoulder or raceway seat. As noted above, the steering tube has had metal machined at points of lower stress or loading to reduce weight without sacrificing strength and safety. The outside diameter of the bearings is about two inches. The upper and lower bearing raceways are further secured in place with an adhesive, preferably an anerobic adhesive but epoxy adhesives can also be used.
Further, according to the invention the handlebar, neck and stem are unitized and designed to accommodate the larger head tube and steer tube discussed above. In a preferred embodiment, the stem is about 13/8 inches in diameter and has a wall thickness of 0.070 inches and a lighter stem and handlebar. This again adds to the positive feel and control on it and the ride is very good notwithstanding the fact that there is reduced flex in the front forks. It is believed that the large flex in the front fork is not necessary because when going over rough terrain and the front wheel for example, hits a bump, the fork being angled towards it the flexible fork will flex backward and in flexing backward bumping the front end of the bike to jack it up in the air in a pogo-stick-like effect. This increases the actual vertical movement over what occurs with a stiff rigid fork as is disclosed in the present application. The stiff fork reduces the degree of bounce so that when you hit a bump, instead of the fork flexing back and raising the front end of the bike and causing it to loose contact with the terrain, the fork does not flex back and the tire seems to deflect more. According to the invention, the tire is made to work harder and the bike stays on track better. That is, the bicycle stays on the ground and control is better and the feel is good and the bicyclist has a feeling of being in control on it, which is very useful. Moreover, the cyclist can go at a higher speed because of having more control, and the traction seems to be better particularly on downhill runs.
The invention has been applied to a mountain bike but it is believed to be just as applicable to road bikes. However, the road bike fork tubing need not be quite as large as the mountain bike, it can be made lighter and use a smaller headset size and smaller blades for cosmetic and air resistance reasons.
The overall effect is to reduce the weight of the front end of the bike by about a pound and one-quarter to about a pound and one-half. The headset is lighter, the front fork is much lighter and the handlebar, neck and stem are likewise lighter. This is due in part to the fact that it is a one-piece handlebar and stem that weighs no more than other high quality stems on the market. It will be noted that the fork according to this invention, will only fit a bike made with the larger head tube. Hence, the invention takes a special frame and a special head tube to adapt to it. Normally new fittings are required. Aircraft torque tube bearings from the bearing assemblies are used. The threads that are needed to adjust the bearings have been eliminated because the bearings are direct press and adhesive secured bearings and no threads are needed. This type bearing adds to the positive feel and control obtained in bicycles according to the invention because they have a lot more rigidity in the torque tube bearings than normal bike bearings have. Placing the bearings inside the head tube strengthens the bearing joints for the head tube.
The present invention deals with the proportional tubing utilized in the fork blades. In the prior art, the blades typically used a straight tube which is the simplest design wherein the uniform wall for each tube increases the wall diameter until the tube has sufficient strength to take all loads and load concentrations. However, this results in a very heavy tube. Butted tubes utilize thin-walled tubes in the center and heavier walls at each end. Stresses are cantilevered at the tube ends where they join with the other tubes. The walls are able to take higher joint stresses while the thinner section allows reduced weight. The reason for proportional tubing, in bicycle frame design, is that the stresses in the tubing cantilevered toward the ends, but the actual working loads are not uniformly distributed around the circumference of each frame tube. In the main frame, in the region of the head tube, the largest loads are the result of high vertical landing loads and head-on impacts. Thus the top and bottom surfaces of the frame tubes see much higher loads than the sides. As known in the art, a more efficient use of material is to reinforce the top and bottoms to special dimensions of the tube (other than round) or increased wall thickness at the top and bottom of the tube, or the combination of the two.
In the front forks, the normal loading includes some torque loads and some side loading. The heaviest loads will come from vertical bumps or head-on impact and/or braking forces. Both vertical and longitudinal forces stress the front fork at a fore/aft cantilever bending motion. In a simple analysis, it appears that increasing the strength of the fork blades and steerer fore and aft cantilever would be the correct approach.
However, it has been discovered through actual testing and detailed analysis that forces in the fork blades are displaced to the side where the steerer attaches, or the wheel or inside of the fork, a small amount. Thus, the optimum here, according to the invention, is not a direct fore and aft reinforcement, but two reinforcements (e.g. thicker wall) are shifted slightly to the inside of the fork as can been seen in the drawings attached hereto. Another factor is that there is a high degree of compressive stress resulting from the vertical bump loading. Thus, in the preferred embodiment, the front of the blades is reinforced a little bit more (e.g., thicker wall and more metal) than the rear because of the straight compressive stress and the cantilevered compressive stresses are cumulative in the front of the blade and oppose each other in the rear. Thus, there is a slight differential in the thicknesses in these areas.